Ceramic honeycombs are widely used as catalytic substrates in automotive, woodstove, and other catalytic applications. The low thermal expansion of these honeycombs makes them excellent thermally shock resistant substrates for high temperature applications.
Generally, thermal shock resistance of a coated substrate is lower than that of a bare ceramic substrate. The thermal expansion coefficient (CTE) of cordierite honeycombs is known to be raised by alumina washcoats, resulting in a significant reduction in thermal shock resistance. It is widely assumed that one of the causes for the increase in CTE is due to penetration of washcoat particles and/or dissolved species into microcracks in the substrate, or precipitation of dissolved material on drying, the size of the penetrating material being generally smaller than the size of the microcracks. Thus, it is essential to block the microcracks during washcoating in order to preserve a high degree of thermal shock resistance.
At present, there are several methods to preserve the thermal shock resistance of washcoated ceramic honeycombs by selectively blocking either the microcracks, surface macropores, or both, prior to application of the washcoat slurry. Several U.S. patents describe the use of various kinds of precoats to reduce the effect of washcoats on CTE, and to improve thermal shock resistance. These patents are briefly described below.
U.S. Pat. No. 4,451,517 relates to catalyst support provided with activated alumina layer formed on the surface of the honeycomb without filling the microcracks with the alumina. This is done by first filling the microcracks with organic material which is subsequently burned away at temperatures lower than the sintering temperature of the activated alumina.
U.S. Pat. No. 4,483,940 relates to a method of manufacture of a honeycomb carrier of enhanced resistance to thermal shocks, which method comprises applying a coat of a water-soluble high-molecular organic compound to the surface of a ceramic honeycomb carrier of monolithic construction and subsequently depositing a catalyst component on the resultant coated carrier. After the washcoat and catalyst are applied, the substrate is fired, during which time the organic coating is burned out. The resulting gap between the substrate and washcoat reduces the "strains" due to the difference in CTE between the washcoat and substrate, resulting in improved thermal shock resistance.
U.S. Pat. No. 4,532,228 relates to depositing an organic material in the microcracks and, preferably, carbonizing the organic material prior to application of the washcoat, which organic material is burned out after application of the washcoat.
There remains a need for methods to improve or preserve the thermal shock resistance of washcoated ceramic bodies, especially honeycombs.